Kurland, Michael. Perchance
Divergences: c 1000 BCE, 1492 CE, 1906 CE
What if: Columbus's first voyage had a fourth ship or the Americas were invaded by Europeans c. 1000 BCE or Germany won an early World War I.
Summary: An apprentice from Philadelphia meets an amnesiac girl who can blip between timelines, and a lot of people are hunting for her.
Published: NAL/Signet 1989 (0451157656).